1. Field of the Invention
The invention concerns a UV-reflective interference layer system for transparent substrates with broadband antireflection in the visible wavelength range, a method for coating a substrate with such a layer system, and the use of such coating systems in various fields of application.
2. Description of the Prior Art
Currently known glass antireflections for the visible spectral range, such as the MIROGARD or the AMIRAN antireflection of Schott-DESAG AG, Grxc3xcnenplan, are interference filters of three layers, wherein a layer with an intermediate index of refraction is first deposited, followed by a layer with high index of refraction, usually TiO2, and then a layer with low index of refraction, usually SiO2 or MgF2. As the layer with intermediate index of refraction, for example, a mixture of SiO2 and TiO2, but also Al2O3 is used. Such three-layer antireflections are deposited, for example, on eyeglass lenses, on monitors, on plate glass, such as display window panels, on treated lenses, etc.
In most instances, these filters have a blue-violet or green residual reflection. When light impinges perpendicularly, the reflection characteristic of glasses coated on both sides is characterized in that the reflection within the wavelength interval of around 400-700 nm is less than 1%, for example, but outside this range the reflection rises to values of up to around 30% (V or W-shaped characteristic), i.e., far above the 8% of uncoated glass.
The drawback to such systems is that, when viewing at an angle that increasingly deviates from the perpendicular, the characteristic shifts to ever shorter wavelengths, so that the long-wave reflection maximum ends up in the visible range, and produces an undesirable red component to the reflected light color.
One goal of the present invention is therefore to find an antireflection whose residual reflection is low in a much broader wavelength range, i.e., in the range from 400 to at least 800 nm with perpendicular incidence of light, and which furthermore also provides broadband antireflection at rather large viewing angles. In many applications, such as display window glazings or glazings for pictures, a neutral-color appearance is in fact desirable, especially for different viewing angles.
Especially for picture glazings, say, in museums, but also in the case of display window glazings, furthermore, it is desirable that an antireflecting glassxe2x80x94if possible, color-neutralxe2x80x94at the same time provides the function of protecting the colors of the picture or the natural or synthetic fibers, as well as the dyestuffs of the window displays, against ultraviolet light.
As is known, the UV component of sunlight or that of lamp light, especially in the case of metal halide or other gas discharge lamps, but also even with halogen bulbs, is sufficient to cause considerable damage over a lengthy period of time, such as discoloration or embrittlement of natural or synthetic fabrics. A UV protection would also be desirable for glazings in office or residential buildings, in order to greatly reduce the fading of wood surfaces, draperies, upholstered furniture, etc., under direct sunlight, and thus enable, for example, an improved passive utilization of solar energy. Present-day thermal protection glasses, which contain a thin silver layer, are not antireflective in the visible range, and furthermore also do not offer sufficient UV protection, since thin silver layers become transparent in the UV.
In the case of known antireflective soft glass, UV protection is achieved by the use of organic polymers as absorbers of UV light, for example, as compound glass, wherein two glass panes are laminated together with a PVB plastic foil adapted by its index of refraction to the glass, for example, 380 xcexcm in thickness (the glass MIROGARD-PROTECT from Schott-DESAG). Such glasses are [used] under intense lamp light, for example, as front panels for lamps, but they are not temperature-stable and they are also degraded by intensive UV radiation. Also, their three-layer antireflection on one side has the above-mentioned limitations, and furthermore the production of compound glass is costly.
Another possibility is the use of UV-absorbing varnish layers, which are several micrometers thick and are transparent to visible light. Such varnish layers are likewise not stable to UV and temperature, and after being deposited on the glass they must further be made antireflective. Regarding the state of the art, refer also to the following publications:
D1: H. Schrxc3x6der, xe2x80x9cOxide Layers Deposited from Organic Solutionsxe2x80x9d, in Physics of Thin Films, Academic Press, New York, London, Vol. 5 (1969), pp. 87-140
D2: H. Schrxc3x6der, Optica Acta 9, 249 (1962)
D3: W. Geffeken, Glastech. Ber. 24, p. 143 (1951)
D4: H. Dislich, E. Hussmann, Thin Solid films 77 (1981), pp. 129-139
D5: N. Arfsten, R. Kaufmann, H. Dislich, Patent DE 3300589 C2
D6: N. Arfsten, B. Lintner, et al., Patent DE 4326947 C1
D7: A. Pein, European Patent 0 438 646 B1
D8: I. Brock, G. Frank, B. Vitt, European Patent 0 300 579 A2
D9: Kienel/Frey (ed.), xe2x80x9cDxc3xcnnschicht-Technologie [Thin layer technology]xe2x80x9d, VDI-Verlag, Dxc3xcsseldorf (1987)
D10: R. A. Hxc3xa4fer, xe2x80x9cOberflxc3xa4chen- und Dxc3xcnnschicht-Technologie [Surface and thin layer technology]xe2x80x9d, Part I, xe2x80x9cCoating of Surfacesxe2x80x9d, Springer-Verlag (1987)
whose disclosure contents are fully incorporated in the present application.
The object of the invention is to specify a coating for a transparent substrate, especially glasses, with which the above-described disadvantages can be overcome.
In particular, one should achieve a UV filtering, on the one hand, without the use of UV or temperature-unstable polymer foils or varnish, and, on the other hand, the antireflection of visible light should be much more broadband and color neutral at the same time.
As regards the UV filtering, one should achieve approximately the same characteristics as for foil or varnish systems.
According to the invention, the object is solved by an interference layer system that comprises at least four individual layers, wherein the consecutive layers have different indices of refraction and the individual layers comprise UV and temperature-stable inorganic materials.
Especially preferred is an interference layer system of five layers with the structure: glass +M1/T1/M2/T2/S, wherein the high-refracting material T has an index of refraction in the range of 1.9-2.3 at a wavelength of 550 nm, the low-refracting material S has an index of refraction between 1.38 and 1.50, and the intermediate-refracting material M has an index of refraction in the range of 1.6-1.8, with layer thicknesses of the individual materials in the ranges of 70 to 100 nm (M1), 30 to 70 nm (T1), 20 to 40 nm (M2), 30 to 50 nm (T2), and 90 to 110 nm (S).
In one embodiment of the invention the highly refractive material is titanium dioxide, the low-refracting material is silicon dioxide, and the intermediate-refracting material is a mixture of these substances.
In an alternative embodiment, instead of titanium dioxide one can also use niobium oxide Nb2O5, tantalum oxide Ta2O5, cerium oxide CeO2, hafnium oxide HfO2, as well as mixtures thereof with titanium dioxide or with each other, as the high-refractive layers; instead of silicon dioxide one can also use magnesium fluoride MgF2 as the low-refractive layer; and instead of Tixe2x80x94Si oxide mixtures one can also use aluminum oxide Al2O3 or zirconium oxide ZrO2 as the intermediate-refractive layers.
As the transparent substrate, in a first embodiment, one can use soft glass in the form of float glass, including a low-iron form.
As an alternative to this, one can also use hard glasses as the substrate, especially aluminosilicate and borosilicate hard glasses or quartz glass.
Besides the interference layer system, the invention also provides a method for applying it onto a substrate.
In a first embodiment of the invention, the individual layers are deposited by means of the dip method or the spin method of sol-gel techniques.
As an alternative to this, the layers can be deposited by cathode sputtering (for example), by physical vaporization, or by chemical gas-phase deposition, especially plasma-supported.
Especially preferred, the interference coatings according to the invention are deposited on transparent substrates comprising an infrared-reflecting thermal protection coating, or transparent layers comprising an interference layer system according to the invention are provided with a thermal protection layer, so that a UV-reflective thermal protection glass is obtained.
Thermal protection glasses are based on the principle of reflection of the infrared heat radiation by a thin, electrically conductive coating that is largely transparent in the visible range. Basically, tin oxide and silver-based layers are considered as heat-reflecting coatings.
Tin oxide can be deposited immediately after the float glass productionxe2x80x94and application of a diffusion-inhibiting SiOx preliminary coatingxe2x80x94in the cool-down phase at around 600xc2x0 C. by means of a spray process. By doping with fluorine or antimony, surface resistances up to 15 Ohms for a layer thickness of around 300 nm are achieved, so that a more than 80% degree of infrared reflection averaged out over the distribution of 300 K thermal radiation is achieved.
As window glazing, therefore, this glass reflects back the majority of the thermal radiation into the space of a building.
The tin oxide deposited by spray pyrolysis during float glass production of interior double-pane glass, for example, must be protected against cleaning, even though it has good mechanical and chemical stability, since substances get worn down due to the relatively high roughness and hardness applied during cleaning processes, and drying is made difficult.
In the double-pane insulated glass composite with an uncoated flat glass pane, these glasses achieve a heat transfer valuexe2x80x94depending on the gas filling and the glass spacingxe2x80x94of up to k=1.6 W/m2K. The drawback is the only moderate visible transmission of 75% of such a double-pane insulated glass for two panes with thickness of 4 mm, which is predominantly attributable to the reflection at the boundary layers. The UV transmission, which should be as low as possible not only when used as glazing for museums or textile shops, but also for residential or office buildings, is 35% in the range of 280 to 380 nm.
Instead of doped tin oxide SnO2:F,Sb, one can also use the transparent semiconductor materials zinc oxide ZnO:Al (aluminum-doped) and indium oxide In2O3:Sn (tin-doped, xe2x80x9cITOxe2x80x9d). Although ITO has a considerably lower electrochemical stability than tin oxide and requires further treatments after the spraying process, zinc oxide cannot be produced with sufficient electrical conductivity by means of a spray process.
Silver-based heat-reflecting coatings achieve significantly more favorable surface resistances down to less than 1 Ohm and, thus, infrared emission levels of 9 to 4%, at the limit down to 2%, so that k-values of 1.1 to 1.4 W/m2K are possible on the basis of such a coated pane in the double-pane insulated glass compound. The visible transmission in this case is at most 76% and if the silver layers are thicker it drops to around 68% for k-values below 1.0 W/m2K. The UV transmission is 36-19%.
The deposition of silver layers is more favorable in terms of thermal reflection, but after the glass production it must be conducted by costly vacuum coating methods, and furthermore additional dielectric layers surrounding the silver layer on both sides and possibly also metal layers to improve the transmission and the long-term stability are required.
A further drawback is that the silver layer composite can only be used on the inside of double-pane insulated glasses, since there is no permanent mechanical or even chemical stability with respect to cleaning processes.
The visible transmission of heat-reflecting insulated glasses, as described above, is inadequate both in the case of tin oxide and for silver-based layers. With an antireflection coating on all four boundary surfaces of a double-pane insulated glass one can obtain glasses whose visible transmission is boosted to 88%. However, the UV transmission is still 25%.
By applying an interference layer system according to the invention, one can obtain thermal protection glasses with low transmission in the UV range and high transmission in the visible range, so-called UV-reflective thermal protection glasses.
Preferably, a UV-reflective thermal protection glass according to the invention comprises an infrared-reflecting thermal protection plate glass, coated with electrically conductive tin oxide, being provided on both sides with a UV-reflective interference layer system, a single pane that is coated on one side with tin oxide and then provided on both sides with the UV-reflective, broadband antireflecting multilayer coating [has] a (mean) visible transmission of 90% or more, as well as a UV transmission (280-380 nm) of 8% or less, while the thermal radiation properties of the tin oxide remain unchanged.
As an alternative to this, UV-reflective thermal protection glasses can be obtained with silver-based, heat-reflecting xe2x80x9clow-exe2x80x9d layers, especially in the form of double-pane insulated glasses. If all the other three glass surfaces except the low-e layer applied to the inside of a glass surface are provided with an interference layer system, the visible transmission increases, for example, from 76% to 85%xe2x80x94for unchanged heat-transfer value kxe2x80x94while the UV transmission is reduced from approximately 30% to around 4%.
If the low-e layer is applied on one side of a plate glass previously made antireflecting on both sides with an interference layer system according to the invention, and combined with a second pane made antireflecting on both sides with an interference layer system as a double-pane insulating glass, the visible transmission is further increased to 87%, while the UV transmission is reduced to 3%.
If the second pane is coated on one side with tin oxide prior to making both sides antireflecting with an interference layer system, the k-value will be reduced by around 0.2 W/m2K, i.e., from 1.0 to 0.8 W/m2K, for example.
The visible transmission of a single pane coated with tin oxide on one side and made antireflecting with an interference layer system on both sides, having a thickness of 4 mm, is 10% higher in absolute terms than that of tin oxide thermal protection glass not made antireflecting, and 2 to 3% higher than that of uncoated float glass. At the same time, the UV transmission is lowered from approximately 45% to 8% without application of polymer varnish or foil.
If one combines a tin oxide-coated single pane provided with the UV-reflective interference layer system on both sides with an identical second pane, the remaining UV transmission will be lowered to 3%, and only a small residue of long-wave radiation will still be admitted in the wavelength region of 360 to 380 nm.
At the same time, the thermal protection properties are significantly improved by the infrared reflection now at two tin oxide layers, and k-values of around 1.2 W/m2K are possible, such as are otherwise achieved only with silver-based thermal protection glasses. The application of a double, IR-reflecting tin oxide layer is only possible because, thanks to the efficient broadband antireflection of all four boundary surfaces with an interference layer system according to the invention, the total visible transmission is around 87% for normal iron-containing float glass with two panes of 4 mm thickness each.
Ifxe2x80x94as has been customary thus farxe2x80x94only one layer of tin oxide is used in a double-pane thermal protection glass, the k-value remains at the minimal 1.6 W/m2K, associated with a somewhat higher visible transmission of 88% and a UV transmission of 4%.